The present invention relates to an apparatus and method for detecting misfiring in an internal combustion engine which can detect misfiring of the engine based on an ion current generated by a spark plug in the space between the electrodes thereof. More particularly, it relates to a misfiring detecting apparatus and method in which a current signal corresponding to the level of ion current thus generated is compared with a threshold so as to determine engine misfiring.
In general, internal combustion engines such as automotive gasoline engines have a plurality (for example four) of cylinders which are operated through four cycles including an intake stroke, a compression stroke, a power stroke and an exhaust stroke. In order to properly control the ignition timing of the cylinders, the order of fuel injection into the cylinders, etc., an engine control unit in the form of a microcomputer is employed for performing various electronic calculations. To this end, based on a cylinder reference position signal representative of the crank positions of the cylinders and a cylinder identification signal which are both generated by a signal generator in synchronism with the rotation of the engine, the microcomputer identifies the operating positions of the cylinders and properly controls their operations.
For example, for control of cylinder ignition, the fuel/air mixture in each cylinder compressed by a piston must be fired for combustion at an optimum timing by a spark generated by a spark plug. In this connection, however, there are times when the mixture in a cylinder, though ignited by a spark plug, does not properly combust depending upon the state of combustion, the condition of the spark plug, etc. In this situation, an abnormal load is applied to the remaining cylinders, giving rise to a fear of engine damage. Thus, in order to maintain safe engine operation, there is a need to detect, for each ignition cycle of each cylinder, whether the mixture in a cylinder has combusted without fail. For such a purpose, a misfiring detecting apparatus has been proposed which can determine the condition of combustion for each cylinder by detecting an ion current which is generated by a spark plug in the space between the electrodes thereof.
FIG. 10 illustrates a typical example of such a known misfiring detecting apparatus for an internal combustion engine. In this figure, a crankshaft 1 of an unillustrated engine is connected with a plurality of pistons received in cylinders (not shown) so that it is driven to rotate in accordance with the operations of the pistons. A camshaft 2 is operatively connected through a timing belt 3 with the crankshaft so as to rotate in synchronism with the rotation of the crankshaft 1.
In the case of normal four-cycle engines, a series of four cycles comprising an intake stroke, a compression stroke, a power stroke and an exhaust stroke are performed for every two revolutions of the crankshaft so that the camshaft, 2 is rotated to perform one revolution per two crankshaft revolutions. Thus, the camshaft 2 rotates one complete revolution for a four-cycle operation of each cylinder in synchronism therewith. As a result, with a four-cylinder engine, the operating positions of the pistons in the cylinder are out of phase with each other by a half revolution (i.e., 180 degrees) of the crankshaft 1 and hence by a quarter of one revolution (i.e., 90 degrees) of the camshaft 2.
A signal generator, which is generally designated by reference character S, includes a rotating shaft 4 connected to the camshaft 2, and a rotating disk 5 fixedly mounted on one end of the rotating shaft 4 for detecting the reference positions of each cylinder. The rotating disk 4 has a plurality (four in the illustrated example) of first windows in the form of arcuate slits 6 formed therethrough, the slits being disposed on a circle around the axis of the rotating shaft 4 and spaced from each other at the same circumferential intervals. Each of the slits 6 has a leading edge and a trailing edge which correspond to prescribed piston positions of a corresponding cylinder. In addition, though not, illustrated, the rotating disk 5 has a second window in the form of a slit formed therethrough, the second slit corresponding to a specific one of the cylinders for identifying it from the remaining ones.
A pair of fixed support plates 8 are provided on the opposite sides of the rotating disk 5. A photocoupler (not shown) including a light emitting diode and a phototransistor is mounted on the opposing support plates 8 on a circle on which the slits 6 in the rotating disk 5 are disposed. Each of the arcuate slits has a leading edge corresponding to a first reference crank position of a corresponding cylinder and a trailing edge corresponding to a second reference crank position thereof in a rotating direction of the rotating disk 5. Each time one of the first slits 6 and the unillustrated second slit in the rotating disk 5 passes between the light emitting diode and the phototransistor of the photocoupler or becomes in alignment therewith during rotation of the rotating disk 5, the photocoupler generates an output signal L containing a train of pulses each of which rises at the leading edge of a slit and falls at the trailing edge thereof.
A controller 10 in the form of an electronic control unit (hereinafter referred to as an ECU) comprising a microcomputer receives the output signal L from the photocoupler as well as other various kinds of signals representative of the operating condition of the engine generated by various sensors (not shown) such as an engine speed sensor, an engine load sensor, etc., so that on the basis of these input signals, it performs various engine control operations such as fuel control, ignition control, etc. For example, based on these signals, the ECU 10 determines the order of operations of the cylinders of the engine, and control s the operations such as ignition of the respective cylinders based on the operating order -thus determined.
A power transistor 11 in the form of an NPN transistor having a grounded emitter is driven to be turned on and off under the control of the ECU 10. An ignition coil 12 has a primary winding connected to a collector of the power transistor 11, and a secondary winding connected to a spark plug 13 through a reverse-current checking diode 14. The power transistor 11, the ignition coil 12, the spark plug 13 and the diode 14 together constitute an ignition means. Although such an ignition means is provided for each of the cylinders, only one of them is exemplarily shown in FIG. 10.
An ion current detector, generally designated by reference numeral 20, is connected between one end of the spark plug 13 and the ECU 10. The ion current detector 20 includes a reverse-current checking diode 21 having a cathode connected to a node between the diode 14 and the spark plug 13, a load resistor 22 having one end thereof connected to an anode of the diode 21, a DC power supply 23 connected in series to the other end of the load resistor 22, a pair of voltage-dividing resistors 24, 25 connected in parallel to a series circuit comprising the load resistor 22 and the DC power supply 23, a capacitor 26 having one end thereof connected to a junction between the diode 21 and the load resistor 22, a comparator 27 having a first negative input terminal connected to a node between the serially connected resistors 24, 25, a second positive input terminal and an output terminal connected to the ECU 10, and a pair of voltage-dividing resistors 28, 29 connected in series to each other between a constant power supply and ground with a node therebetween being connected to the second input terminal of the comparator 27 for supplying it with a constant threshold voltage TH. The voltage-dividing resistors 24, 25 together constitute a voltage generating means for generating a voltage V corresponding to an ion current I. Also, the voltage-dividing resistors 28, 29 together constitute a threshold generating means for generating a constant threshold voltage which is applied as a combustion determining reference to the second input terminal of the comparator 27.
The operation of the known misfiring detecting apparatus as constructed above will now be described in detail. As the crankshaft 1 rotates, the rotating disk 5 is driven to rotate through the intermediary of the timing belt 3, the camshaft 2 and the rotary shaft 4. During rotation of the rotating disk 5, the photocoupler (not shown) mounted on the support plates 8 generates an output signal L as the first slits 6 and the unillustrated second slit pass between the light emitting diode and the phototransistor (not shown) mounted on the opposing support plates 8. The signal L thus generated includes a crank angle reference signal representative of predetermined crank positions of each cylinder and a cylinder identification signal for identifying a specific one of the cylinders. The crank angle reference signal contains a train of pulses each of which rises at the leading edge of a slit 6 corresponding to a first reference crank position (e.g., 75 degrees before top dead center (BTDC)) of a corresponding cylinder and falls at the trailing edge thereof corresponding to a second crank angle position (e.g., 5 degrees BTDC) of the cylinder. For example, the first reference crank position is a control reference such as a power-supply starting time at which the power supply to the ignition coil 12 is started by the ECU 10, and the second reference crank angle position is another control reference such as an ignition time at which the power supply to the ignition coil 12 is cut off for causing the spark plug 13 to generate a spark. The cylinder identification signal contains a pulse corresponding to the specific cylinder which is generated, for example, at the time when a pulse of the crank angle reference signal corresponding to the specific cylinder is generated. The signal L of the signal generator S as well as other signals representative of the operating condition of the engine are input to the ECU 10. Some examples of the other signals are an engine rotation signal representative of the number of revolutions per minute of the engine, and an engine load signal representative of the engine load or the throttle opening.
Based on the signal L, the ECU 10 identifies the operating order or states of the respective cylinders, and it generates and distributes an ignition control signal to the power transistors 11 for the corresponding cylinders at proper timing according to the operating order thereof thus determined. As a result, one of the power transistors 11 is turned on by the ignition control signal from the ECU 10, so that a current starts to flow from the power supply to ground through the primary winding of the ignition coil 12 and the now conductive power transistor 11. After current has been supplied to the primary winding of the ignition coil 12 for a predetermined period of time, the ECU 10 stops generating the ignition control signal, thus turning off the power transistor 11. As a result, a high voltage is developed across the secondary winding of the ignition coil 12 to cause the spark plug 13 to generate a spark. Then, the voltage applied to the ignition coil 12 by the power supply, which is a negative voltage, is interrupted immediately after the discharge of the spark plug 13.
Immediately after the discharge or sparking of the spark plug 13 causing explosive combustion of an air/fuel mixture near the spark plug 13, there will develop a great number of positive ions in the limited space between the electrodes of the spark plug 13, generating an ion current I therearound. The ion current I thus generated by the positive ions is attracted to the negative electrode of the spark plug 13 and thence flows to the negative electrode of the DC power supply 23 by way of the diode 21 and the load resistor 22. As a result, the ion current I generates a voltage across the load resistor 22 which is changed into a voltage V by the voltage-dividing resistors 24, 25 and then fed to the first negative input terminal of the comparator 27. The voltage V input to the comparator 27 corresponds to and is proportional to the ion current I. That is, the voltage V becomes high when explosion or combustion takes place whereas it becomes low in the absence of combustion. On the other hand, the second positive input terminal of the comparator 27 is supplied with a threshold voltage TH which is set to a predetermined constant value by the voltage-dividing resistors 28, 29. Accordingly, the comparator 27 generates a low level output when the voltage V is less than the threshold TH, and a high level output when the volt, age V is equal to or greater than the threshold TH. That is, the comparator 27 outputs an ON signal to the ECU 10 only when it detects an ion current I.
Based on the voltage V representative of the ion current I thus detected and the operating order or states of the cylinders identified from the signal L, the ECU 10 determines whether normal combustion has taken place in the cylinder which had been fired.
If the fired cylinder is normal or normal combustion is taking place in the firing cylinder due to discharge or sparking of the corresponding spark plug 11, a number of positive ions are thereby generated in the limited space between the electrodes of the spark plug 13. If, however, there is no explosion or combustion for some reason, there will be substantially no positive ions generated. As a result, the ECU 10 can determine the condition of combustion or misfiring in the fired cylinder on the basis of the voltage V and the identified operating order or states of the cylinders.
In this connection, however, there is a tendency for noise having a short pulse width to be superposed on the ion current I particularly at the time of ignition for example, so that the level or voltage V of the ion current is thereby raised. Accordingly, if the condition of combustion is determined based solely on the result of such a comparison, there is a possibility that the comparator 27 will generate an output C representative of an ion current of a high level. In this case, the ECU 10 would determine that normal combustion has taken place in a fired cylinder, despite the fact that no combustion has actually occurred therein. This may lead to engine damage, as referred to above.
With the known misfiring detecting apparatus as described above, in which it is determined that combustion has taken place when the level V of ion current exceeds the threshold TH, there is a defect that in the event that noise greater than the threshold TH is momentarily applied, incorrect determination is made in spite of the occurrence of a misfire. In addition, among many ignition cycles or power strokes, misfiring will sometimes accidentally happen without any particular abnormality in engine operation, so it is not practical to make the determination of misfiring (i.e., abnormality in the engine) based on a single detection or a few detections of misfiring which might occur by chance. Thus, it is difficult to perform highly reliable determination of misfiring in the cylinders at all times.
Moreover, though it is a general measure to stop the supply of fuel to a cylinder which is determined to be misfiring, irrespective of the state or extent of misfiring, such a measure is not always effective for treating misfiring since relatively light or not so serious misfiring situations can be remedied to recover normal combustion by taking other suitable measures such as increasing ignition energy and the like.